Resonant cavity magnetron having built-in magnetic pole shoes with an electrically conductive metal coating to prevent flash-over to the cathode



March 5, 1968 J. A. VAN DE GOOR A 3,372,299

RESONANT CAVITY MAGNETRON HAVING BUILT-IN MAGNETIC POLE SHOES WITH AN ELECTRICALLY CONDUCTIVE METAL COATING T0 PREVENT FLASH-OVER TO THE CATHODE Filed Sept. l5, 1964 2 Sheets-Sheet 2 INVENTOR.

JOHANNES A. VAN DE GOOR ive/6% .especially the case when the United States Patent ABSTRACT OF THE DISCLOSURE In a magnetron having built-in substantially conical perforated pole-shoes between which the ends of a cathode extend, at least the surface of the edge of the poleshoes facing the c-athode consists of refractory, electrically conductive metal such as molybdenum to prevent iashover and reduce sputtering and evaporation of cathode material.

The invention relates to a resonant cavity magnetron having built-in substantially conical perforated pole shoes, the ends ofthe cathode reaching between these pole shoes.

The ends of the cathode reaching between the pole shoes must conduct away, by conduction and radiation, the heat produced in the cathode by electrons striking the cathode when the tube oscillates.

Notwithstanding the fact that the emission of the cathode is limited as much as possible to the part 'lying between the resonant cavities in the anode block by providing end rings on the cathode and applying emission-deteriorating layers to the cathode parts outside these rings, flash-over occurs between the ends of the pole shoes and the cathode due to the form of the electric lield. This is tube is activated, but also during the further life of the tube. As a result of this flash. over, especially coarser particles of the pole shoes are sputtered towards the resonant cavities and the cathode.

Part of -the iron of the pole shoes impinging on the cathode evaporates again towards the resonant cavities. Thisv is especially the case if the cathode is constituted by a porous tungsten body containing barium as the activating material. For such cathodes have a high operating temperature than conventional oxide cathodes, that is to say an operating temperature -of at a maximum 1,350 C. The iron reaching the resonant cavities through the cathode reduces to a considerable extent the Q-factor thereof, so that it becomes more diticult for the magnetron to oscillate. The coarser iron particles arriving directly in the resonant cavities exert their influence mainly by shortcircuit.

In order to improve the vacuum-tightness of the pole shoes, it is already known to apply a layer of copper to these parts by sintering, if desired in combination with nickel. However, the copper disappears rapidly so that the Q-factor ofthe resonant cavities is again reduced by nickel or iron. v

The invention has for its object to provide an improved structure.

In a resonant cavity magnetron having built-in substantially conical perforated pole shoes between which reach the ends of the cathode, according to the invention, at least the surface of the edge of the pole shoes facing the cathode consists of a satisfactorily conducting higlh-melting point metal having a thickness of at least a few tenths of a millimeter. Ac-cording to the invention, the metal is preferably molybdenum, but the requirements imposed in accordance with the invention are also satisfied by platinurn and tungsten. The edge of the pole shoes may be coated with a molybdenum layer either by the application of a foil by rolling or by the application of a layer by sintering or by Schoops treatment to the profiled edge; alternatively, the pole shoe may have soldered to it a comparatively thick metal ring of the desired proiile. As a result ofthe high melting temperature of the edge of the pole shoe in accordance with the invention, the sputtering is reduced and the material sprayed on the cathode will no longer evaporate.

The invention will now be described more fully with reference to the accompanying drawing, in which:

FIG. 1 is a partial section of a resonant cavity magnetron in accordance with the invention, and

FIG. 2 is a section of a pole shoe according to another embodiment.

Referring now to FIG. l, reference numeral 1 denotes the copper outer wall of a magnetron the anode vanes of which are designated by 2. Copper boundaries of the end spaces are denoted by 3 and the iron pole shoes by 4. Between the pole shoes, the cathode is disposed which consists of a central molybdenum body 5, 6 in the center of which a porous tungsten body 7 is enclosed which is impregnated with barium aluminate. The molybdenum body is provided with end rings 8 which, just as the ends of the molybdenum body 5, 6 reaching between the pole shoes, are coated by sintering with a zirconium layer 9. The zirconium layer 9 applied by sintering has two functions: viz. to counteract ther-mal emission and to improve the heat radiation of the cathode ends. A molybdenum tube 10 of 0.3 mm. thickness is applied by rolling to the parts of the tip facing the cathode yand to the inner side of the bore of the pole shoes 4. The molybdenum has a greater resistance to the influence of flash-over occurring from the cathode towards the pole shoes than the iron of the pole shoes itself.

In FIG. 2, reference numeral 11 designates the iron pole shoe having a molybdenum edge 12. The molybdenum is soldered in the form of a ilat ring to the not yet completely sized pole shoe, whereupon the desired profile is realized by milling.

What is claimed is:

1. A resonant cavity magnetron comprising a hollow anode defining a plurality of resonant cavities, a pair of substantially conical perforated pole-shoes integrally connected to said anode, and a cathode extending between the pole-shoes and having end surfaces opposite edge surface portions of the pole-shoes, said edge surface portions of the pole-shoes opposite the cathode being constituted of an electrically conductive high melting point metal selected from the group consisting of molybdenum, tungsten and platinum having a thickness of at least several tenths of ya millimeter to prevent flash-over between the pole shoes and the cathode.

2. A resonant cavity magnetron comprising a hollow anode defining a plurality of resonant cavities, a pair of substantially conical perforated pole-shoes integrally connected to said anode, a cathode extending between the pole-shoes and having end surfaces opposite edge surface portions of said pole-shoes and a layer of electrically conductive high melting point metal selected from the group consisting of molybdenum, tungsten and platinum having a thickness of at least several tenths of a millimeter covering said edge surface portions of said pole-shoes opposite said cathode to prevent flash-over between the pole-shoes and the cathode.

3. A resonant cavity magnetron comprising a hollow anode defining a plurality of resonant cavities, a pair 0f substantially conical perforated pole-shoes integrally connected to said anode, a cathode extending between the 3 i pole-shoes and having end surfaces opposite edge surface References Cited vpofrtions yof said pole-shoes, and a profiled ring-shaped UNITED STATES PATENTS selected from the group consistingsof molybdenum, tunglayer of a hlgh rneltlng-polnt electncally conductlve metal 2,748,067 5/1956 Pease et al. 313 -159 X sten and vplatinum having a thickness of at least several 5 S CHATMON, Assistant Exmer kof said p0le-shoes `opposite said cathode to prevent asht th f `11' 't `d d f t en s Q a m1 une er coverlng sa1 e ge sur ace por 1ons ELI LIEBERMAN, Examiner.

xver between the pole-shoes and the cathode. S. CHATMON, Assistant Examiner. 

